1. Technical Field
The embodiments herein generally relate to electrical amplifiers, and, more particularly, to differential low noise amplifiers.
2. Description of the Related Art
Conventional low noise amplifiers can have instrumentation amplifier architectures and differential difference amplifier architectures. A block diagram of an instrumentation amplifier architecture is shown in FIG. 1(A). FIG. 1(B) illustrates one possible circuit implementation of the architecture of FIG. 1(A). In this architecture, two identical amplifiers Ap and An are used to provide capacitive input impedance to the Inp and Inn terminal and sense the voltage difference across its input terminals and provides gain to their respective output. With the feedback loop the output is regulated to be:
      V    out    =            (              1        +                  Zf          Zi                    )        ⁢                  V                  i          ⁢                                          ⁢          n                    .      
In FIG. 1(B), a traditional two stage opamp is used as illustration and from basic opamp theory, it is known that the noise of the opamp is dominated by the first stage devices, which are devices 101-105, 201-205, as the second stage device noise is attenuated by the gain of the first stage and thus can be neglected for most practical cases. In addition to the four passive components Zip,Zin and Zfp,Zfn, there are four input active devices 101, 102, 201, 202, four active load devices 103, 104, 203, 204, and two tail current devices 105, 205 that contributes to the total input noise. The detailed noise voltage spectral density equation is as follows and here it is assumed that the device noise primarily consists of thermal noise and flicker noise:
            V              noise        ,        total            2        =                  V                  device          ,                      thermal            -            noise                          2            +              V                  device          ,                      flicker            -            noise                          2            +              V                  passive          ,                      thermal            -            noise                          2                        V              device        ,                  thermal          -          noise                    2        =                  4        ·                              4            ⁢            kT            ⁢                                                  ⁢            γ                                gm            101                              +              4        ·                              4            ⁢            kT            ⁢                                                  ⁢                          γ              ·                              gm                103                                                          gm            101            2                              +              2        ·                              4            ⁢            kT            ⁢                                                  ⁢                          γ              ·                              gm                105                                                          gm            101            2                                          V              device        ,                  flicker          -          noise                    2        =                  4        ·                  K                                                    (                WL                )                            101                        ⁢                          C              OX                        ⁢            f                              +              4        ⁢                              K            ·                          gm              103              2                                                                          (                WL                )                            103                        ⁢                          C              OX                        ⁢                          f              ·                              gm                101                2                                                        +              2        ·                              K            ·                          gm              105              2                                                                          (                WL                )                            105                        ⁢                          C              OX                        ⁢                          f              ·                              gm                101                2                                                                    V              passive        ,                  thermal          -          noise                    2        =                  2        ·                  V                      noise            ,            Zip                    2                    +              2        ·                              V                          noise              ,              Zfp                        2                                G            2                              
Here, G stands for the gain of the amplifier and is determined by
      G    =          1      +                        Z          fb                          Z                      i            ⁢                                                  ⁢            n                                ;gm stands for the transconductance of the metal oxide semiconductor (MOS) device; K stands for the flicker noise constant; Cox stands for the oxide thickness; f stands for frequency; and γ stands for the thermal noise constant. In practice, devices 101-104 are configured to have the same width and length and are biased with same current. Apart from manufacturing non-idealities it can be assumed that devices 101-104 are, to a first order, identical to each other. The same applies to device groups of (103, 104, 203, 204), (105, 205), (Zip, Zin), and (Zfp, Zfn). As noise is summed in the power domain, to achieve a given noise requirement, all devices have to be low in noise to meet the requirement and this mandates a lot of power to be consumed in both amplifiers Ap and An to meet a low level of noise floor.
The second approach is the differential difference amplifier. In this architecture, instead of two single ended amplifiers, a differential amplifier with two differential pair is used. However since the two terminal input to each differential pair is not exactly balanced depending on the loop gain, the tail current device still contributes to the total noise budget. In FIG. 1(B), elements 106, 206 are output stage bias transistors; elements 107, 207 are transistors in the output stage that provides gain; elements 108, 208 are zero canceling resistors or transistors for stability; and elements 109, 209 are Miller compensation capacitors. The diagram and an example of implementation of this architecture is shown in FIGS. 2(A) and 2(B), respectively. Four active input devices (221-224), two active load devices (225-226), two tail current devices (227-228), and four passive components (Zip, Zin, Zfp, Zfn) are identified that contribute to the majority of the total input noise. In FIG. 2(B), elements 229, 230 are output stage bias transistors; elements 231, 232 are transistors that provide gain in the output stage; elements 233, 236 are Miller compensation capacitors for stability; and elements 234, 235 are zero canceling resistors/transistors for stability. If the noise analysis that was performed for the instrumentation amplifier is repeated, the following noise equations are obtained:
            V              noise        ,        total            2        =                  V                  device          ,                      thermal            -            noise                          2            +              V                  device          ,                      flicker            -            noise                          2            +              V                  passive          ,                      thermal            -            noise                          2                        V              device        ,                  thermal          -          noise                    2        =                  4        ·                              4            ⁢            kT            ⁢                                                  ⁢            γ                                gm            221                              +              2        ·                              4            ⁢            kT            ⁢                                                  ⁢                          γ              ·                              gm                225                                                          gm            221            2                              +              2        ·                              4            ⁢            kT            ⁢                                                  ⁢                          γ              ·                              gm                227                                                          gm            221            2                                          V              device        ,                  flicker          -          noise                    2        =                  4        ·                  K                                                    (                WL                )                            221                        ⁢                          C              OX                        ⁢            f                              +              2        ⁢                              K            ·                          gm              225              2                                                                          (                WL                )                            225                        ⁢                          C              OX                        ⁢                          f              ·                              gm                221                2                                                        +              2        ·                              K            ·                          gm              227              2                                                                          (                WL                )                            227                        ⁢                          C              OX                        ⁢                          f              ·                              gm                221                2                                                                    V              passive        ,                  thermal          -          noise                    2        =                  2        ·                  V                      noise            ,            Zip                    2                    +              2        ·                              V                          noise              ,              Zfp                        2                                G            2                              
However, irrespective of which architecture is used, the conventional techniques tend to suffer from relatively high noise levels, common mode stability problems, and tend to require increased power levels for operation. Moreover, the conventional techniques, as indicated above, tend to require four active input devices. Accordingly, there remains a need for a new low noise, low power amplifier.